1. Field of the Invention
This invention relates to floating point amplifiers and more particularly to a compound floating point amplifier having a first section which uses level and slew to make one relatively large gain change or not, and stores the output in a sample and hold circuit which provides the input to a simple binary gain amplifier.
2. Description of the Prior Art
Seismic data acquisition systems must deal with a very wide range of input signal levels. Dynamic range is defined as a ratio of the maximum signal that can be applied, without exceeding the distortion specification, to the noise level. This ratio is usually expressed in decibels whereas dB=20 log ratio.
When dynamite is used as a seismic source, the initial signal level may be several hundred millivolts. A few seconds later the signal may be less than a microvolt. A range requirement for a seismic data acquisition system using dynamite therefore is in excess of 120 dB. Typical prior art systems obtain 100 to 115 dB range.
Most seismic data acquisition systems have an analog to digital (A/D) converter for converting the data, the A/D converters having a dynamic range of approximately 90 dB. Modern seismic systems generally use a floating point amplifier to extend the dynamic range. In these amplifiers, the gain is set to a value that will give a nearly full scale signal to the A/D converter. The steps are in powers of two with the majority using four to one or two to one gain steps. A few use eight to one or 16 to one gain steps. These floating point amplifiers use the signal to determine the required gain level. Ideally an amplifier that makes four to one steps would keep the output signal between 1/4 and full scale of the A/D converter. In practical use, a safety margin is allowed to insure that no signal exceeds A/D converter full scale.
In present seismic data acquisition systems, a plurality of data channels are multiplexed to a single floating point amplifier and A/D converter. This arrangement requires a very fast acting amplifier with a wide bandwidth. The function of the amplifier is to receive the wide dynamic range of input signal and to reduce the dynamic range to be less than the dynamic range of the associated A/D converter. In actual practice, the range is reduced to a small value causing the A/D converter to operate near the upper limit of its dynamic range so that the converted signals will have a high resolution. The floating point amplifier generates a digital gain word that is associated with the digital data to enable an associated computer to determine the true amplitude of the input signals.
At present, the most widely used floating point amplifier in the seismic industry is the predictive type amplifier which, before making a gain decision, predicts the amplitude that this input signal will actually have when a samaple is taken. Typically, the amplifier takes a sample of the signal level "E". As an example, approximately 1.25 microseconds later, it measures the rate of change "A" which is multiplied by the time "t" until the A/D converter completes taking a sample. "At" is the amount the signal will change. Various gain increases are made depending upon the combination between "At" and "E". For example, if E+At is less than 18% of full scale and E is less than 22% of full scale, a four to one gain increase is made. If this is not true, no gain step will be made. The process is immediately repeated. If the above conditions are met, another four to one increase of gain will be made. If E+At is greater than 80% of full scale or if E is greater than 88 % of full scale, a four to one decrease is made. If E+At is between 20% and 80%, and E is less than 88% of full scale, no gain change is made. This type of amplifier is proven to be very good, but it is complicated, expensive, consumes considerable power and is physically fairly large.
Another type of prior art floating point amplifier makes use of a sample and hold circuit that provides an input to the amplifier. The sample and hold circuit takes a sample of the input signal and then holds the sample at a fixed value while the amplifier gain ranges. This amplifier has a fixed signal level and is therefore less complicated and less expensive than the predictive amplifier. However, the dynamic range of the sample and hold circuit is relatively small and reduces the dynamic range of operation of this type amplifier.
This invention achieves the dynamic range of the predictive amplifier with a substantial reduction in complexity and cost.